XC/marathon project 26/29er

Among my world-tour bike project, I'd like to build a bike for XC/marathon racing and 12/24h events. I need a frame I could use geared or single speed with 80/100mm or rigid forks. I prefer steel frames, I'd be happy with a light(er) one. I'm 182cm and have 80cm inseam length.

What about a 29er? Is that worth to start a completly new build? Or should I use my 26er Inbred 853 with Rohloff or Alfine hub?

Do You know any test result which show difference in speed, hrm, cadence, time ridden on 26/29ers on the same tracks?